This invention relates generally to increasing the defeat resistance of magnetically actuated sensing devices and more particularly to improving the capability to monitor the position of doors and windows blocking unauthorized access to restricted areas and preventing unauthorized operation of machinery when safety devices have been disabled.
Magnetically actuated switches previously used in security monitoring systems have typically utilized a magnetically actuated reed switch mounted, for example, in the frame of a doorway or window, with conductors leading therefrom to a security system control unit, and one or more magnets mounted on the edge of the door or window for actuating the reed switch. When the actuating magnet approaches the reed switch within a certain distance, determined by the sensitivity of the reed switch and the strength of the actuating magnet, the magnet actuates the reed switch by closing a set of magnetic contacts therein, which sends an electrical signal to a control unit.
In such applications it is desirable to utilize a relatively sensitive switch so that the distance between the switch and its actuating magnet necessary to actuate the switch is not critical, thereby permitting slight variations in the position of the magnet without changing the state of the switch and setting off a false alarm. A previously known improvement on such magnetically-actuated reed switches for increasing their sensitivity uses a small permanent magnet placed close to a reed switch to bias the response of the switch to the presence of an external magnetic field, so that a weaker magnetic switch field is required to actuate the magnet. Adjustable biasing may be achieved, for example, by variation of the distance or angular relationship between the magnet and the longitudinal axis of the reed of a switch, as shown in Nicholls U.S. Pat. No. 3,974,469, and by varying the location of the reed along an imaginary axis parallel to the axis of polarity of the biasing magnet, as shown by Tann U.S. Pat. No. 3,305,805.
Another desirable feature of switches used in security systems is resistance to manipulation or deception by the use of foreign magnetic fields. High-security switches known as "balanced" switches have been developed for this purpose. An example of this "balanced" type of switch device is the model DR-850 switch manufactured by Walter Kidde & Co. of Belleville, N.J. which has two single-pole-double-throw reed switches and a biasing magnet associated with one of them. An actuating permanent magnet is provided whose position is adjustable within its housing so that when properly adjusted the reed switches will be operated to produce a "normal" indication. However, any attempt to defect the system with an externally applied magnet, regardless of its field direction, is alleged to upset the balance of the switches and thereby produce an "abnormal" indication. Such "balanced" high-security switches, however, must be carefully adjusted during their installation to provide proper actuation.
Adjustment of the position of the actuating magnet relative to the switch is critical in the prior art devices adapted for use in security systems, and when seasonal changes in air temperature and humidity cause changes in the alignment of a door to its frame, minor misalignment is often sufficient to cause the device to produce false indications, requiring expensive service calls for realignment of the switch and its actuating magnet.
To increase the adaptability of monitoring system to changing actuator positions, Holce U.S. Pat. No. 4,210,889, uses a biasing magnet for each of three reed switches to bias each switch to the same respective "open" or "closed" position. To increase the defeat resistance of the monitoring system, the relative magnetic polarity of each biasing magnet is alternated in the switching unit. Another defeat resistant monitoring system using biasing magnets is the level 3 safety interlock switch manufactured by Sentrol Co. of Portland, Oreg., assignees of the above-referenced Holce patent. Such high-security switches, however, are still difficult to adjust during installation and in addition require complex switching circuitry. The level 3 switch uses two unbiased parallel reed switches in addition and at an angle to the three biased reed switches of Holce '889, for a total of five reed switches. These switches require an actuator with three magnets of alternating polarity.
Holce and other previously mentioned patents improve the defeat resistance of monitoring systems but do not operate effectively under the unique conditions associated with interlock systems. For example, monitoring systems determine whether or not a door or window has been opened. If a door is opened while the security monitoring system is on, an alarm is activated. Thus, to defeat a monitoring system each switch must be held in a closed position, without activating the alarm, while opening the door. In a safety interlock circuit, however, the goal of the defeat is to cause the switching unit to activate (e.g. to close all switches), with the door already open so the machine can be run in an unsafe condition, that is, with the guard door open. Interlock systems are, therefore, inherently easier to defeat than monitoring systems. For example, in a monitoring system, if an intruder attempts to defeat the system by placing a foreign magnetic field next to the switching unit, he has no indication whether the magnet properly disabled the system. In a interlock system, however, the intruder is notified of a disabled system by the activation of the interlocked machinery. The intruder can thereby move the foreign magnetic field around the switching unit until he properly disables the system. Thus, a switching unit that is highly defeat resistant in a monitoring system may be easily defeated in a safety interlock application. The Holce '889 patent can be readily defeated with persistence. The current Sentrol level 3 unit with 5 reed switches cannot be defeated but has some problems with residual magnetism and crosstalk of magnetic fields which can sometimes lead to latchup, leaving the switch in an actuated condition when the actuator is removed.
In addition, mechanical interlock systems typically operate under harsher environmental conditions than typical monitoring systems. For example, interlock systems are used in mechanical equipment that is frequently dented, bent, and worn to a greater degree than the doors and windows where monitoring systems are typically attached. Also, interlock circuits must operate in dirt, grease, and other contaminants that may alter typical circuit operation. Therefore, an interlock system must be able to operate under a more extreme range of environmental conditions with an actuator that must operate properly at a wider range of distances and alignment configurations.
Accordingly, a need remains for simple defeat resistant magnetic interlock/monitoring system that is adaptable to changes in actuator position and operating environment.